[Writeup] CSAW2020 - Baby Mult

Friday. September 11, 2020 - 1 min

Challenge

Solution

We’re given some values that range from 0 to 255 and the file is called program.txt so it’s probably shellcode

so we convert these values to hex and store them in a file using any hex editor.

now we need to convert it to readable disassembly so i ran ndisasm -b32 x but the instructions didn’t make any sense so i tried ndisasm -b64 x to view x64 assmebly instead of x86

now the instructions are pretty straightforward since they’re mainly imul and not idiv :’D, the program calculates each part of the flag by multiplying two or more values and stores them in a local variable.

you can calculate the flag using a python script or you can compile the binary and let gdb do the work for you

here’s x.asm

Compiling

nasm -f elf64 x.asm to assemble the program,  gcc x.o -nostdlib -o x to compile the object file nasm produced into an executable file, -nostdlib because gcc will link the standard C libraries by default which already contain a _start that invokes main entry point and then complain that it has multiple entry points:’D, chmod +x x to make it executable.

Debugging:

gdb -q x -ex "start" -ex "b *0x000055555555431d" -ex "c" to let the program finish execution

x/s $rbp-0x80

x/s $rbp-0x88

x/s $rbp-0x90

Flag

flag{sup3r_v4l1d_pr0gr4m}

coreflood

Just another reverse engineer.